Guided-wave-based defect inspection is known for its long-range inspection ability and high sensitivity in detecting small defects in objects. In major applications, guided-wave-based defect inspection is used for detecting cracks and corrosion that may occur in structures like pipes, rods, strands, cables, etc.
Existing sensors used for creating (exciting) and receiving guided-wave generally are magnetostrictive sensors that include a hard sensing coil for generating a necessary magnetic field for exciting the desired guided-wave. FIG. 1A shows a conventional hard coil-based magnetostrictive sensor 10 arranged around a wire strand 1. The magnetostrictive sensor 10 includes three consecutive groups of hard coils 11A, 11B, 11C disposed along the wire strand 1 and each surrounding the wire strand 1. The hard coils 11A-11C are formed by hard coil wires. The longitudinal length of each group is equal to half of the wavelength of the excited guided-wave mode. The first and third groups of hard coils 11A, 11C are wound in a first direction (e.g., clockwise), and the second group 11B disposed between the first and third groups 11A, 11C is wound in a second, opposite direction (e.g., anti-clockwise), as shown in FIG. 1B. The magnetostrictive sensor 10 also includes two sets of permanent magnets 12 disposed around the wire strand 1. The first set of permanent magnets formed by three equally-angularly spaced magnets is arranged at the end of the first group of coil 11A, the second set of permanent magnetics formed by three equally-angularly spaced magnets is arranged at the end of the third group of coil 11C. The hard coils 11A-11C are arranged between the two sets of permanent magnets 12. Yokes 13 are arranged to connect magnets of the two sets 12 that are at substantially the same angular position. As shown in FIG. 1A, three yokes 13 are provided and they are equally angularly spaced around the wire strand. Referring to FIG. 1B, the magnets and yoke generate a uniformly distributed bias magnetic field on the steel strand. The coils on the other hand, generate dynamic magnetic fields. At the instance shown in FIG. 1B, the first and third groups of coils 11A, 11C generate magnetic field that has the same direction as the bias magnetic field while the second group of coil 11B generate a magnetic field that has an opposite detection to the bias magnetic field. The bias magnetic field and the dynamic magnetic fields are both substantially parallel to a longitudinal axis of the strand 1. Through the interactions between the bias and dynamic magnetic fields, the desired longitudinal guided-wave mode can be excited in the strand 1 for inspection.
While this type of hard coil-based magnetostrictive sensor is useful in some applications, they are usually bulky, and are only suitable for use in steel strands of predetermined size. Mounting of the coils or the magnets may be difficult if the other objects to be measured do not have a suitably sized free end. Also, applications are also limited to use in inspection of metallic objects or structures.